Disturbances in endothelial cell (EC) barrier regulation is a hallmark of lung inflammation and are critically dependent upon rearrangements in the EC cytoskeleton (microfilaments and microtubules). We have previously shown the involvement of actomyosin-driven microfilament contraction and increase in myosin light chain (MLC) phosphorylation in the receptor agonists-mediated permeability. In contrast, information about the role of microtubule network in EC barrier regulation is limited. Our novel observations indicate that thrombin- and TGFb-mediated EC barrier dysfunction is dependent (in part) upon microtubule remodeling as stabilization of microtubule network significantly attenuates thrombin- and TGFb-induced permeability. Our data demonstrate strong correlation between agonist-induced microtubule dissolution and increased phosphorylation of several microtubule-associated proteins (MAPs) including calmodulin (CaM)-dependent kinase II (CaMKII) targets, tau and filamin, implicating a possible role of MAPs and CaM-dependent enzymes such as CaMKII and MLC kinase (MLCK)in receptor agonist-mediated microtubule remodeling. We have recently shown that microtubule inhibitors significantly increases EC MLC phosphorylation and permeability indicating the importance of microtubules in maintaning the EC barrier. Barrier dysfunction induced by microtubule dissolution is significantly attenuated by Rho and p38 MAP kinase pathways inhibition. Together, these data clearly convey the complex involvement of microtubule-mediated pathways in the regulation of EC permeability. The overall objective of this proposal is to examine the molecular mechanisms by which microtubules contribute to the receptor agonist-mediated EC barrier dysfunction under static and cyclic stretch-stimulating conditions. SA#1 will examine the role of CaMKII in thrombin and TGFb-induced microtubule dissolution, SA#2 will characterize the involvement of MLCK in agonist-mediated microtubule remodeling, SA#3 will examine the role of Rho pathway in microtubule-mediated increases in MLC phosphorylation and permeability, SA#4 will identify p38 MAPK pathways involved in microtubulemediatedEC contractility and barrier dysfunction, SA#5 will established the link between sphingosine 1-phosphate barrier-protective signaling and receptor agonist-mediated microtubule dissolution. These studies will provide an understanding of novel signaling pathways involved in lung EC barrier regulation.